1. Field of the Invention
The present invention relates to a variable valve timing apparatus and more particularly to a variable valve timing apparatus having a mechanism changing an opening/closing timing of a valve at an amount of change according to an operation amount of an actuator.
2. Description of the Background Art
VVT (Variable Valve Timing) has conventionally been known that changes the phase (crank angle) in (at) which an intake valve or an exhaust valve is opened/closed, according to an operating condition. Generally, a variable valve timing apparatus changes the phase by rotating, relative to a sprocket or the like, a camshaft that drives the intake valve or exhaust valve to open/close. The camshaft is rotated by such an actuator as hydraulic or electric motor.
Such a variable valve timing apparatus may be operated not only at the time of engine operation but also at the time of engine stop to change a valve timing (camshaft phase). Specifically, in the case where the valve timing at the time of engine stop differs from the valve phase suitable for the next engine start, the valve timing is changed by the variable valve timing apparatus during engine stop, in preparation for the next engine start (for example, Patent Documents 1-4).
Patent Document 1 (Japanese Patent Laying-Open No. 2003-184585) discloses that, immediately after the automatic stop of the engine, the valve opening/closing conditions (valve lift amount, valve timing, and the like) are controlled so that the conditions are suitable for the next engine automatic starting, which are estimated based on the coolant temperature and the like at this point of time, and an operation of a variable valve lift mechanism or the like is thereafter stopped. On the other hand, Patent Document 2 (Japanese Patent Laying-Open No. 2005-180307) and Patent Document 3 (Japanese Patent Laying-Open No. 2005-146993) discloses a valve timing control apparatus in which a rotational phase is naturally returned to an intermediate phase between the most retarded angle phase and the most advanced angle phase at the time of inertial rotation during engine start or after engine stop whereby the rotational phase at which the engine can be started can be set at the intermediate phase.
Further, Patent Document 4 (Japanese Patent Laying-Open No. 2004-156508) discloses a valve timing control apparatus that changes a valve timing to the angular position suitable for the next engine start by supplying electric current to a hysteresis brake as an electromagnetic actuator for a prescribed period of time after turning off an ignition key, namely after engine stop.
In general, operation energy to an actuator for a variable valve timing apparatus at the time of engine stop is supplied from a secondary battery charged when the engine operates. Therefore, in the configuration in which the actuator is operated to change the valve timing at the time of engine stop, the power consumption therefore should be restrained. However, Patent Documents 1-3 do not mention the power consumption of the actuator in changing the valve timing at the time of engine stop.
On the other hand, the valve timing control apparatus disclosed in Patent Document 4 can prevent exhaustion of the battery to some extent by limiting the period of power supply to the electromagnetic actuator (hysteresis brake) after engine stop to a certain range. However, although the amount of change of the valve timing required at the time of engine stop differs depending on the valve timing at the time of engine stop, the aforementioned power supply period is fixedly set and therefore power supply to the electromagnetic actuator is continued even in the period after the valve timing is changed to the valve timing suitable for the next engine start, possibly resulting in unnecessary power consumption.